Unpin desktop task in overview

Desktop task should behave like any other task in overview and should
not show up as a large tile.

Bug: 309008406
Flag: ACONFIG com.android.window.flags.enable_desktop_windowing_mode DEVELOPMENT
Test: Swipe fullscreen task into overview
      Open overview from home
      Swipe freeform task into overview
Test: Image test will be added in b/319237536
Change-Id: Iefca85747de7595357813d5ed999df0e4b51120a